“Net Zero Energy” Isn’t All It Seems

The hot new idea in energy and real estate is the “zero net energy building.” It usually means a building with enough solar panels on the roof so that over the course of a year, it produces as much energy as it consumes.

And that means the building poses no burden on the grid, right?

Well, no. In fact, the grid’s work may get harder when a zero net energy building is connected . And it means that in real life, the building still has a carbon footprint.

That’s not a fatal flaw for “zero” buildings or for solar on the roof. In fact, many aspects of a zero net energy building are unambiguously good and ought to be incorporated into a lot of structures – good insulation, high efficiency lighting and other devices, and placement of the building to make optimum use of the sun, for example.

And there’s a certain attractiveness to coming out even in the energy equation, like the squirrel who spends the fall gathering all the acorns he will consume through the winter.

But it’s only energy, and the building doesn’t run on just energy. It runs on a combination of energy and power. The graph above shows the power part. The purple area, above the line, shows how much power the house is demanding from the grid. The green area, below the line, shows how much power it is sending to the grid, which is electricity from the solar panels, minus household use at that instant. The grid, formerly a supplier, is now a supplier and a customer, and if the flows from the customer to the utility are large enough, the grid must be re-configured to accept them.

While the house may come out even in energy terms, it still imposes a power burden on the utility company.

Energy and power are both aspects of electricity, and the terms are frequently used interchangeably, but they should not be. Energy is typically measured in kilowatt-hours, which is a quantity. Power, also called “capacity” in the electric power industry – measured in kilowatts, is an instantaneous measurement, like speed.